Because virtually every informed and experienced teacher was aware of the simple reality that the cane worked with a great many boys and hardly ever did them any harm if used appropriately. They also knew that it worked far less effectively with girls, and was more likely to do harm. Nowadays, we do actually have published research that backs this up, but with this as in many other cases, experienced teachers at the chalk face didn’t need research to tell them what commonsense already had.
If girls have never been caned in the same number as boys how can you draw on the overall effectiveness, if you are caning less girls then there could be larger number of girls it could be effective on